1. Field of the Invention
The present invention relates to a zoom lens barrel.
2. Description of the Related Art
FIGS. 2 and 3 show the conventional zoom lens barrel in general use.
Numeral 102 denotes a cam barrel in which curved cam grooves 102a and 102b are formed; these grooves being engaged with a plurality of pins 100b and 101b These pins 100b and 101b are embedded in retaining rings 100a and 101a which respectively retain variator lens unit 100 and compensator lens unit 101. The cam barrel 102 is rotated by rotating an operation ring 102c for zooming. Numeral 103 denotes a fixed barrel in which straight grooves 103a and 103b are formed, which grooves 103a and 103b are for the variator and compensator lens unit 100 and 101, respectively.
With the construction described above, when the operation ring 102c is rotated, the pins 100b and 101b shift along the straight grooves 103a and 103b arranged in the fixed barrel 103, whereby a desired zooming is achieved.
FIG. 2 (B) is a development of some cam grooves as seen from direction C in FIG. 2 (A); and FIG. 2 (C) is a cross-sectional view taken along the line D--D of FIG. 2 (A).
As illustrated in FIG. 2 (A), a clearance .delta. is usually provided in the optical axis direction so that the cam barrel 102, in which the curved cam grooves 102a and 102b are formed, may smoothly rotate with respect to the fixed barrel 103.
Because of the clearance .delta., however, when the cam barrel 102 shifts from the state shown in FIG. 2(A) to the state shown in FIG. 3(A), in the optical axis direction due to, for example, an external shock, the variator lens unit 100 and the compensator lens unit 101 shift along the straight grooves 103a and 103b formed in the fixed barrel 103, thereby shifting out of a desired zooming shift track. The optical performance thus deteriorates. This deterioration becomes pronounced especially when the zoom ratio is high and a zoom lens with a long focal length is used, thereby producing poorly projected images. The cam grooves 102a and 102b indicated by the two-dotted broken lines in FIG. 3 (B) show those grooves in the positions as illustrated in FIG. 2(B).
In the above case, the shift amount of a rotary barrel in the optical axis direction can be rendered small either by setting a small clearance .delta., or by precisely setting a clearance .delta.. However, the torque for rotating the cam barrel 102 then becomes large. When the cam barrel 102 is electrically rotated, a larger motor is required, thereby increasing power loss.